A significant portion of the population has experiences pain or discomfort resulting from spinal injuries or degenerative conditions in and around the vertebral discs. While many individuals may simply experience minor sprains or strains that may be somewhat limiting, numerous individuals may develop severe lower back pain caused by inflammatory changes in the lumbar disc associated with such changes.
A spinal segment includes a lumbar disc and two facet joints. Degenerative changes in the disc can lead to changes in the facet joint, and vice versa. In order to treat a degenerative condition and to alleviate the pain involved with such a malady, surgical methods may be employed to replace the degenerative component of the spinal segment, such as the damaged disc. However, the replacement of a degenerative disc may not suffice, as the facet joint components of the spinal column may still be a source of discomfort and/or limited mobility. As such, it may desirable to replace a degenerative or problematic facet joint with a posterior stabilization device.
In addition, the degenerative process may result in a condition called spinal stenosis, where there is a narrowing of the spinal canal. This is caused by a combination of reduced disc height, ligamentum hypertrophy, a forward slip of the vertebra and disc bulging. Surgery is sometime needed to deal with this condition, where a surgical procedure may involve decompression of the spine and/or the removal of posterior portions of the spinal column. This often makes the spine unstable, requiring stabilization of the spine after the decompression. Such stabilization may be achieved with an instrumented postero-lateral fusion, where pedicle screws are inserted into the vertebra to be fused and connected with rods or plates, and bone is laid on the side of the spine over the transverse processes. Stabilization may also be accomplished by a dynamic stabilization device where there is no need to add a fusion, and the device stabilizes the spine. A posterior dynamic stabilization device is typically attached to pedicle screws inserted in to the vertebrae, however, conventional devices are limited in their allowable range of motion and results may vary from patient to patient. It would be desirable to allow the surgeon to select or otherwise adjust the amount of motion desired in the stabilization device based on the requirements of the individual patient.
However, each motion segment of the spine moves around an instantaneous center of rotation. The instantaneous axis of rotation is the axis perpendicular to the plane of motion passing through a point in the body that does not move. For each spinal motion segment this point is basically the point at which the motion segment rotates about, also termed the center of rotation. If the spine is altered in any way, such as with disc and facet degeneration or spinal implantation, this center of rotation shifts, leading to undesirable consequences, such as further degradation, limited movement ability, etc.
Accordingly, it would be desirable to provide a stabilization device which results in a minimum reduction in the natural movement of a motion segment of the spinal column when implanted, thereby reducing any additional strain on the adjacent level of the spinal segment of the individual receiving the stabilization device. Moreover, it would be desirable to provide a stabilization device able to resist and/or dampen the flexion and extension forces experienced by the motion segment during movement, while providing limited motion in the desired directions. It would also be desirable to provide a stabilization device having an adjustable path of motion that can be selectively adjusted for a particular patient or application. In addition, it would further be desirable to provide a stabilization device which can continuously adjust to a moving centre of rotation of a disc prosthesis, or, if used on its own, with that of the lumbar disc anteriorly.